This application claims priority to South Korean Application No. 2001-24413 filed May 4, 2001.
1. Field of the Invention
The present invention relates to directional rolling pendulum seismic isolation systems and roller assembly therefor, and more particularly, to directional rolling pendulum seismic isolation systems and roller assembly therefor, that can reduce seismic load applied to structures, such as bridges, general buildings, precision machines or cultural assets.
2. Description of the Related Art
In traditional earthquake resistant design of structures, the structural members, components and systems are required to have adequate amount strength and ductility in the event of strong earthquakes. However, the structures designed according to this strength design principle tend to experience severe damage or excessive deformation in the event of very strong earthquake even though they may not collapse. Therefore alternative methods have been developed that can protect structures from earthquakes within predetermined deformation limit. One of the most widely used protection methods is seismic isolation system. Because it has been proved to be very effective in the reduction of seismic load in recent earthquakes, the use of seismic isolation systems is on an increasing trend.
A Korean patent application No. 2000-37760 discloses a basic principle of the seismic isolation systems. The above basic principle will be explained again in the following.
If a structure 201 is fixed to the ground 202 as shown in FIG. 1a, it can be modeled as a single degree of freedom system as shown in FIG. 1b. The response of the structure to the earthquake action, such as base shear force and relative displacement can be estimated using response spectra.
FIGS. 2a and 2b show graphs of acceleration response spectra and graphs of displacement response spectra respectively as examples. The drawings show response spectra for two values of damping ratio. In the graph of FIG. 2a, the vertical axis indicates the spectral acceleration and the horizontal axis indicates the period. In the graph of FIG. 2b, the vertical axis indicates the spectral displacement and the horizontal axis indicates the period. The base shear force acting between the structure and the ground by the horizontal ground motion can be estimated from the acceleration response spectrum shown in FIG. 2a. That is, if the natural period and the damping ratio ("xgr"1 or "xgr"2) of the single degree of freedom are given, the spectral acceleration is read from the curves shown in FIG. 2a. If the obtained spectral acceleration value is multiplied by the mass of the structure, the base shear force is approximately found.
The relative displacement between the superstructure and the ground can be estimated from the displacement response spectrum shown in FIG. 2b. If the natural period of the single degree of freedom and the damping ratio are given, the spectral displacement is read from the curves shown in FIG. 2b. The obtained spectral displacement shows the displacement of the single degree of freedom relative to the ground.
As can be seen from the graph shown in FIG. 2a, generally, if the period becomes longer, the spectral acceleration is reduced. Moreover, in the same period, if the damping ratio becomes larger, the value of the spectral acceleration is reduced.
In the case of the spectral displacement, as can be seen from the graph shown in FIG. 2b, if the period becomes longer, the relative displacement is increased. Furthermore, in the same period, if the damping ratio becomes larger, the value of the spectral displacement is reduced.
In conclusion, if the period is longer and the damping ratio is higher, the spectral acceleration is reduced, and thereby the seismic force, i.e., floor shear force, becomes small. The seismic isolation systems adopt the above mechanical principle. For example, the seismic isolation system such as a high damping lead rubber bearing has mechanical properties that the horizontal stiffness is very small but the damping capacity is high.
As shown in FIG. 3a, if a seismic isolation system 203 is installed between the base frame and a ground 202, the natural period of the whole structural system becomes even longer, and also the damping ratio increases. Like this, if the natural period T becomes longer period Te or the damping ratio "xgr" is increased to a ratio "xgr"e then the seismic force can be reduced significantly, as can be seen from the graph shown in FIG. 3b. 
However, as shown in FIG. 3c, if the natural period becomes longer, the relative displacement increases. To restrict the increase of the relative displacement, dampers can be installed in addition to the conventional seismic isolation system having low damping capacity. One of the seismic isolation systems having high damping capacity and the long natural period, which do not require the additional dampers, is a sliding pendulum seismic isolation system. However, the sliding pendulum seismic isolation system used presently has a structure that a slider moves on a dish having a concave surface, and therefore if the seismic isolating period becomes longer, the diameter of the dish becomes even larger. In the case of bridges, generally, an area to install a seismic isolator on a pier or an abutment is extremely restricted.
It is required to lengthen a seismic isolating period and maintain a low friction coefficient in structures, which may be easily damaged even by a low seismic load, such as precision machines or cultural assets. However, it is difficult to lengthen the seismic isolating period sufficiently if a general lead rubber bearing is used because the precision machines or the cultural assets are lower in weight than general structures. Otherwise, in the case of conventional pendulum seismic isolation systems, it is possible to lengthen the seismic isolating period, but it is difficult to maintain the friction coefficient in a low condition. Furthermore, the conventional pendulum seismic isolation systems have another problem that the sliding surface must have a larger diameter if the period is lengthened. The conventional pendulum seismic isolation systems utilizes measures such as injecting lubricating oil into the surface of a friction plate or applying special coating to the sliding surface to lower the friction coefficient. Therefore, to protect the structures, which are light in weight and may be easily damaged even by the low seismic load, such as precision machines or cultural assets, from a seismic tremor, a new type of seismic isolation systems, which can lengthen the seismic isolating period and maintain the friction coefficient in the low condition in an easy and stable manner, has been required.
It is, therefore, an object of the present invention to provide a pendulum seismic isolation system having a new configuration, which can be easily installed without limitations in an installation area.
It is a another object of the present invention to provide a pendulum seismic isolation system, which moves in predetermined directions and yet effectively induces seismic isolation effects in all horizontal directions for the earthquake motion that is applied in arbitrary direction.
It is a further object of the present invention to provide a pendulum seismic isolation systems suitable for structures, which may be easily damaged even by a low seismic load, such as precision machines, cultural assets and buildings requiring a long seismic isolating period to isolate seismic force in a restricted space while having advantages of the conventional pendulum seismic isolation systems.
To achieve the above objects, the present invention provides a directional rolling pendulum seismic isolation system, which reduces earthquake effects on the structures using pendulum motion in selected directions.
The present invention provides bi-directional rolling pendulum seismic isolation systems for reducing seismic force acting on a structure by rolling pendulum movements, each system comprising a lower plate forming a rolling path in a first direction; an upper plate forming a rolling path in a second direction; and a roller assembly performing a pendulum motion by rolling and moving along the lower and upper plates; wherein the roller assembly performs the pendulum motion when seismic load is applied, thereby reducing the seismic load of a structure.
According to the embodiment of the present invention, the upper and lower plates have upper and lower channels, on which the roller assembly rolls and moves, respectively, and the roller assembly includes a main body, a plurality of lower rollers mounted on a lower portion of the main body, the lower rollers rolling and moving along the lower channel of the lower plate, and a plurality of upper rollers mounted on an upper portion of the main body, the upper rollers rolling and moving along the upper channel of the upper plate.
Further, in another embodiment of the present invention, the roller assembly includes a lower main body on which a plurality of lower rollers are mounted on a lower portion thereof and an upper main body on which a plurality of upper rollers are mounted on an upper portion thereof, the lower rollers rolling and moving along the lower channel of the lower plate, the upper rollers rolling and moving along the upper channel of the upper plate, and elastic or elasto-plastic objects being inserted between the upper main body and the lower main body. Thus, the roller assembly is manufactured in a separable type.
In the above embodiment, preferably, the elastic or elasto-plastic objects of the separable roller assembly are spheres, which have a prescribed elasticity and damping property, and the upper and lower main bodies respectively have hemispherical holes for inserting the elastic or elasto-plastic objects.
Further, in the above embodiment, preferably, the upper main body and the lower main body are able to rotate with respect to a vertical axis. Especially, the elastic or elasto-plastic objects of the separable roller assembly may be spheres, which have a prescribed elasticity and damping property, and the upper and lower main bodies respectively may have central hemispherical holes for inserting the elastic or elasto-plastic objects and outer holes formed around the central holes.
Otherwise, the upper and lower main bodies respectively may have central hemispherical holes and outer holes formed around the central holes, and the elastic or elasto-plastic objects of the sphere type, which have a prescribed elasticity and damping property, may be inserted into the central holes. Further, the elastic or elasto-plastic objects of a doughnut type, which have a prescribed elasticity and damping property, may be inserted into the outer holes.
In another embodiment, the elastic or elasto-plastic objects of the separable roller assembly may be spheres, which have a prescribed elasticity and damping property, and the upper and lower main bodies respectively may have holes for inserting the elastic or elasto-plastic objects of a disc type.
Further, in another embodiment, preferably, an intermediate main body may be inserted between the upper main body and the lower main body, and the upper main body and the lower main body are rotated relative to the intermediate main body in a horizontal direction respectively. Thus, the systems are manufactured in an articulated type.
According to another embodiment of the present invention, the roller assembly has a prescribed ratio of breath/height (B/H) to prevent an overturn when performing the pendulum motion, and a radius of curvature (rL) of a circular section of the upper channel is smaller than that of the first directional pendulum motion to prevent the upper rollers from being separated from the upper channel while the roller assembly performs the pendulum motion in the lower channel. Further, a radius of curvature (rT) of a circular section of the lower channel is smaller than that of the second directional pendulum motion to prevent the lower rollers from being separated from the lower channel while the roller assembly performs the pendulum motion in the upper channel, and thereby performing a stable seismic isolation function without overturn or separation from the lower channel or the upper channel while the roller assembly performs the bi-directional pendulum motion.